Hydraulic drive mechanisms for mobile vehicles are specially used for mobile machinery such as excavators, wheel loaders or also graders in which an internal combustion engine drives a hydraulic pump the flow rate of which supplies the hydraulic motors with pressure medium which, in turn, are connected with drive wheels which they drive. Axial piston motors of variable displacement are often used in order to change the rotational speed of the drive wheels. But in the axial piston motors, the maximum rotational speed is limited and to exceed the rotational speed would damage the motor.
U.S. Pat. No. 4,069,886 discloses a hydraulic drive mechanism for a mobile vehicle in which the front wheels can be driven via a respective motor and the rear wheels are directly driven by the internal combustion engine via a mechanical transmission. Since the rotational speed of the hydraulic motors is limited, the front wheels can be used only within a limited working range and above the working range have to be separated from the vehicle wheel via mechanical clutches. The motors are for this purpose entirely uncoupled mechanically. It is thereby possible, via the drive system proper of the vehicle, to operate the front wheels at a higher rotational speed than the admissible rotational speed of the hydromotors. The hydromotors are stationary here.
DE 39 07 633 C2 discloses a hydraulic drive mechanism for a mobile vehicle which serves as exclusive drive system; one internal combustion engine driving a hydraulic pump the flow rate of which supplies two hydromotors with pressure medium connectable with an output shaft via a reduction gear. Since the hydromotors are connected with the output shaft via different reduction gears, there is danger that one hydromotor is operated beyond its maximum rotational speed. For this reason, one clutch is disposed between the output shaft and one hydromotor in order that the hydromotor can be mechanically uncoupled.
The problem on which this invention is based is to provide a hydraulic drive mechanism for mobile vehicles in which at least one hydraulic motor drives one output shaft and also can remain connected with the output shaft even above a maximum rotational speed of the motor by the motor being operable above its maximum rotational speed.